A DOM tree is rather a pain to implement in practice. You need fast foo.childNodes[i] and also fast foo.nextChild. And fast inserts and removes. And APIs that use (parent, offset) pairs as their basic concept (like ranges) and other APIs that use (parent, prevSibling) pairs (like node insertion). Oh, and memory use needs to be minimal. And the code needs to have low constants, not just low algorithmic complexity.